1. Field of the Invention
The present invention relates to a liquid crystal display device and a method for producing the same. More specifically, the present invention relates to an inexpensive liquid crystal display device having excellent display quality and a simple method for producing the same.
2. Description of the Related Art
A liquid crystal display device in a twisted nematic (TN) made is conventionally used for a variety of applications (e.g., a plane display for a personal computer, a wordprocessor, amusement equipment and a television set, or a display plate, a window, a door or a wall utilizing a shuttering affect). The viewing angle characteristics of the TN mode liquid crystal display device will be described with reference to FIGS. 32A to 32C. FIGS. 32A to 32C axe schematic cross sectional views showing an orientation state of liquid crystal molecules in the TN mode liquid crystal display device. FIG. 32A shows a state under application of no voltage to the liquid crystal. FIG. 32B shows a state under application of a voltage to the liquid crystal so as to display a gray scale. FIG. 32C shows a state under application of a saturated voltage to the liquid crystal.
In the TN mode liquid crystal display device, as shown in FIGS. 32A to 32C, when a voltage is applied to a liquid crystal layer 3 intarposed between substrates 1 and 2, liquid crystal molecules in the liquid crystal layer are oriented. In the case where the liquid crystal molecules are oriented in the TN mode, in the gray scale state shown in FIG. 32B, an apparent transmittance of the liquid crystal molecules when viewed from the direction A is different from that when viewed from the direction B. As a result, the direction dependence of the viewing angle characteristics occurs (e.g., in the case where a viewer observes in different directions A and B, display contrast is significantly different). In order to improve the direction dependence of the viewing angle characteristics, a liquid crystal display device in a wide viewing angle mode was proposed where the liquid crystal molecules are oriented in at least two directions in a pixel.
The improvement of the viewing angle characteristics in the liquid crystal display device in the wide viewing angle mode will be described with reference to FIGS. 32D to 32F. FIGS. 32D to 32F are schematic cross sectional views showing an orientation state of liquid crystal molecules in the wide viewing angle mode liquid crystal display device. FIG. 32D shows a state under application of no voltage to the liquid crystal. FIG. 32E shows a state under application of a voltage to the liquid crystal so as to display a gray scale. FIG. 32F shows a state under application of a saturated voltage to the liquid crystal.
In the wide viewing angle mode liquid crystal display device, as shown in FIG. 32D, a liquid crystal layer 3 has a liquid crystal region 8 and a polymer region 7 surrounding the liquid crystal region. In this type of liquid crystal display device, in the gray scale display state shown in FIG. 32E, liquid crystal molecules 9 in the liquid crystal region 8 are axisymmetrically (e.g., concentrically, radially and spirally) oriented around an axis 10. Therefore, an apparent transmittance of the liquid crystal molecules when viewed from the direction A and an apparent transmittance when viewed from the direction B are averaged so as to be substantially equal. As a result, the direction dependence of the viewing angle characteristic is improved compared with the TN mode liquid crystal display device.
As specific examples of the wide viewing angle mode liquid crystal display devices, known are the following seven types of liquid crystal display devices.
A first type of liquid crystal display device is the one shown in FIGS. 32D to 32F. This type of liquid crystal display device includes a liquid crystal region surrounded by a polymer region (e.g., a polymer wall) in a liquid crystal cell. Furthermore, this liquid crystal display device does not require a polarizing plate, nor an orientation treatment. In this liquid crystal display device, a transparent state and an opaque state are electrically controlled utilizing a birefringence of the liquid crystal. In such a liquid crystal display device, the ordinary light refractive index of the liquid crystal molecules is basically matched with the refractive index of a supporting medium (polymer in the polymer region). Thus, the liquid crystal display device displays the transparent state where the liquid crystal molecules are uniformly oriented under application of a voltage, and displays a light scattering state due to a random orientation of the liquid crystal molecules under application of no voltage. This type of liquid crystal display device is produced in such a manner that a mixture of liquid crystal and a photocurable resin or a thermosetting resin is injected between a liquid crystal call, and the resin in the mixture is cured so as to deposit the liquid crystal and thus form a liquid crystal region in tho resin (polymer wall) This method is disclosed in Japanese National Patent Publication No. 61-502128. Furthermore, Japanese Laid-Open Patent Publication Nos. 4-338923 and 4-212928 disclose another technique for obtaining a wide viewing angle mode, where polarizing plates are provided on both sides of the liquid crystal display device so that the polarization axes thereof are orthogonal to each other.
A second type of liquid crystal display device, which is a non-scattering type, employs a polarization plate. This liquid crystal display device includes a liquid crystal region composed of a plurality of domains surrounded by polymers, which is formed by phase separation of a mixture of liquid crystal and a photocurable resin (Japanese Laid-Open Patent Publication No. 5-27242). In such a liquid crystal display device, the orientation state of each domain of the liquid crystal region is disturbed by the polymer formed by phase separation so as to be a random state. As a result, since a direction to which the liquid crystal molecules in each domain rise is different from that for another domain under application of a voltage, .DELTA.n.multidot.d is averaged so that the apparent transmittances when viewed from any direction become equal. Therefore, the viewing angle characteristics in the gray scale are improved.
A third type of liquid crystal display device has, on the surface of a substrate, a film formed from crystalline polymer having a spherulite structure. In such a liquid crystal display device, the liquid crystal molecules in a liquid crystal region are oriented utilizing an axisymmetric orientation regulation force of the spherulite structure so as to realize the wide viewing angle display mode (Japanese Laid-Open Patent Publication No. 6-308496).
In a forth type of liquid crystal display device, an alignment film is applied onto a substrate without performing an alignment treatment such as rubbing, so that the liquid crystal molecules are randomly oriented (Japanese Laid-Open Patent Publication No. 6-194655).
In the third and forth types of liquid crystal display devices, since the liquid crystal molecules in a pixel are oriented in different directions, it is likely that disclination lines are generated and contrast deteriorates.
In order to prevent the generation of the disclination lines in the pixel, a fifth type of liquid crystal display device has been proposed. In this liquid crystal display device, the liquid crystal molecules in the pixel are axisymmetrically oriented. For example, the Applicant has proposed in Japanese Laid-Open Patent Publication No. 7-120728 a liquid crystal display device which is produced by irradiating the liquid crystal cell having a liquid crystal material and a photocurable resin with light in a controlled manner (e.g., irradiating the liquid crystal call with light via a photomask). In such a liquid crystal display device, the liquid crystal molecules are axisymmetrically (e.g., spirally) oriented in the pixel region under application of no voltage, and when a voltage is applied to the liquid crystal molecules, the spiral orientation is converted into a homeotropic state. As a result, the viewing angle characteristics are significantly improved.
In a sixth type of liquid crystal display device, the axisymmetric orientation of the liquid crystal molecules is realized by an orientation treatment (e.g., by forming an axisymmetric narrow groove in a substrate) (Japanese Laid-Open Patent Publication Nos. 6-265902 and 6-324337).
The fifth type of liquid crystal display device is conceptual and poor in its practicability. In the sixth type of liquid crystal display device, it is difficult to control pretilt of the liquid crystal molecules. Therefore, it is likely that disclination lines are generated. Moreover, the stability of the axisymmetric orientation is insufficient.
A seventh type of liquid crystal display device has a so-called ASM structure. In this liquid crystal display device, a liquid crystal cell including a liquid crystal material and a photocurable resin is irradiated with light while changing the temperature and the applied voltage in accordance with a specific rule. Thus, the axisymmetric orientation of the liquid crystal molecules in a pixel is realized (e.g., Japanese Laid-Open Patent Publication Nos. 6-301015 and 7-120728).
However, in the case where the liquid crystal molecules are axisymmetrically oriented by the aforementioned techniques, the position of the axis of the axisymmetric orientation is not sufficiently controlled. Therefore, the orientation axis of the axisymmetric orientation of the liquid crystal molecules inclines, or the position of the orientation axis is dislocated. A problem of axis dislocation will be described with reference to FIGS. 33A and 33B. FIGS. 33A and 33B are schematic views showing the states observed with a polarizing microscope when the liquid crystal cell is inclined under crossed Nicols. FIG. 33A shows the case where the axes are not dislocated. FIG. 33B shows the case where some axes are dislocated. As understood from the comparison between FIGS. 33A and 33D, since an average transmittance in a pixel where the axis is dislocated is different from that in other pixels, when the entire screen is viewed, roughness is observed. Furthermore, when observed as the viewing angle is changed, an area of a portion which looks black is increased in a pixel.
As described above, in the conventional liquid crystal display devices, the position of the orientation axis of the axisymmetric orientation is not sufficiently controlled. Furthermore, in the conventional liquid crystal display devices, a complex production process is required even if it results in an axisymmetric orientation of the liquid crystal molecules having the orientation axis which is insufficiently controlled in its position (e.g., it is necessary to irradiate the liquid crystal cell with light while changing the temperature and the applied voltage in accordance with a specific rule). In other words, the conventional liquid crystal display devices have problems associated with high production costs and insufficient position control of the orientation axis.
Therefore, a liquid crystal display device which does not require a complex production process (i.e., an inexpensive liquid crystal display device) and where the position of the axis of the orientation of the liquid crystal molecules can be precisely controlled (i.e., a liquid crystal display device having an excellent display quality (e.g., excellent viewing angle characteristics and no roughness)) is required.